Numerous types of lumber products have been manufactured by a process where composite assemblies of wood products are coated with an adhesive, and thereafter subjected to compression and heat to form the pressed composite assembly. For example, this technique is used to manufacture particle board from small wood particles and plywood from wood veneer sheets.
A process has recently been developed for manufacturing structural wood products from long, relatively thin strands of wood by coating the strands with an adhesive, arranging the strands side-by-side in a lengthwise dimension of the lumber product and subjecting the arranged strands to heat and compression. By this technique, a high strength dimensioned wood product can be formed. An example of such a process is disclosed in U.S. Pat. No. 4,061,819.
Belt presses, typically used in processes for the manufacture of composite wood products are shown, inter alia, in U.S. Pat. Nos. 3,120,862; 3,723,230; 3,792,953, 3,851,685; 3,993,426; 4,043,732 and 4,213,748. The belt presses are comprised, for example, of facing endless belts between which the material is compressed, and platens and antifriction devices which hold the belts in pressure engagement with the material. In these prior art compression techniques, the inlet end of the press belts, and the platens over which they run, converge toward one another to form a compressing zone.
It has been determined that within the compressing zone of a continuous press, strands are generally free to move with respect to one another for a short period of time. As the belts continue to converge, the strands are no longer free to move but, rather, have positions set with respect to one another. This setting of relative positions can be referred to as "lock-up." After lock-up occurs, further convergence of the press belts only causes further compression of the material. Since lock-up occurs in a converging area, the material being pressed is not in a planar disposition, but rather in a curved disposition. This curved disposition occurs in two opposite directions about a reference plane passing between the belts. Since the material has locked up, the material cannot shift into a planar relationship, rather, the material is forced from this curved disposition into its final planar form. Following passage through the converging portion of the belts, i.e., the compressing zone, the compressed product generally passes through a compression zone in which the belts of the press are parallel.
It has been discovered that a significant part of the curvature of the strands at lock-up remains or is remembered as an internal stress in the pressed composite assembly. When the assembly is a generally thin planar object, such as plywood or particle board sheets, such internal stresses do not present a problem. However, when relatively thick assemblies are manufactured, for example, dimensioned lumber made of wood strands, the internal stresses can present a problem because such thick assemblies may be cut horizontally, thereby releasing the internal stress. Thus, when the lumber product is cut horizontally, the two halves bow in opposite directions.
An additional internal stress problem, which occurs in a continuous process of forming dimensioned lumber products from thin wood strands, such as the product disclosed in U.S. Pat. No. 4,061,819, is a result of the manner in which the strands are arranged prior to their entry into the belt press. As wood strands are aligned to one another in a longitudinal direction and successive layers of strands are laid upon one another, the strands do not rest level upon a preceding strand, but rather a forward end of one strand rests upon a rearward end of a preceding strand. This results in a build-up of strands at an angle above the horizontal. This staggered, overlapping relationship can be referred to as "card decking" because it is similar to the manner in which cards would lay upon one another when they are spread out on a flat surface from a stacked deck. This card decking or angular build-up of the strands results in an internal stress in the dimensioned lumber product produced. Since the build-up occurs in one direction, the stress results in a bowing effect in one direction.
The method and pressed composite assembly of the present invention have been developed to compensate in various ways for these internal stress problems.